Add JEST_WORKER_ID to runner environment (#58)

Co-authored-by: fisker <lionkay@gmail.com>

Author: silverwind

README.md

@@ -16,6 +16,7 @@ The lists below are not comprehensive: feel free to [start a discussion](https:/
 - Inline and external snapshots
 - Jest cli options: `--testNamePattern`/`-t`, `--maxWorkers`, `--runInBand`
 - Jest config options: `setupFiles`, `setupFilesAfterEnv`, `snapshotSerializers`, `maxWorkers`, `snapshotFormat`, `snapshotResolver`, `slowTestThreshold`
+- Jest environment variables: `JEST_WORKER_ID`
 
 ### Unsupported Jest features
 

src/worker-runner.js

@@ -4,7 +4,7 @@ import { performance } from "perf_hooks";
 import * as snapshot from "jest-snapshot";
 import { jestExpect as expect } from "@jest/expect";
 import * as circus from "jest-circus";
-import Tinypool from "tinypool";
+import Tinypool, { workerId } from "tinypool";
 
 /** @typedef {{ failures: number, passes: number, pending: number, start: number, end: number }} Stats */
 /** @typedef {{ ancestors: string[], title: string, duration: number, errors: Error[], skipped: boolean }} InternalTestResult */
@@ -15,12 +15,15 @@ const initialSetup = once(async projectConfig => {
   // We can "polyfill" it for process.cwd() usage, but it
   // won't affect path.* and fs.* functions.
   if (Tinypool.isWorkerThread) {
+    process.env.JEST_WORKER_ID = String(workerId);
     const startCwd = process.cwd();
     let cwd = startCwd;
     process.cwd = () => cwd;
     process.chdir = dir => {
       cwd = path.resolve(cwd, dir);
     };
+  } else {
+    process.env.JEST_WORKER_ID = "1";
   }
 
   for (const setupFile of projectConfig.setupFiles) {